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Modem

by Dennis Faas on September, 18 2005 at 08:09AM EDT

A modem (a portmanteau word constructed from modulator and demodulator) is a device that modulates an analog carrier signal (sound) to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data.

The most familiar example of a modem turns the digital '1s and 0s' of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone System (POTS), and once received on the other side, converts those sounds back into 1s and 0s.

Modem: History

Modems were first introduced as a part of the SAGE air-defense system in the 1950s, connecting terminals located at various airbases, radar sites and command-and-control centers to the SAGE director centers scattered around the US and Canada. SAGE ran on dedicated communications lines, but the devices at either end were otherwise similar in concept to today's modems. IBM was the primary contractor for both the computers and the modems used in the SAGE system.

By the early 1960s commercial computer use had bloomed, due in no small part to the developments above, and in 1962 AT&T released the first commercial modem, the Bell 103. Using frequency-shift keying, where two tones are used to represent the 1's and 0's of digital data, the 103 had a transmission rate of 300 bit/s. Only a short time later they released the Bell 212 modem, switching to the more reliable phase-shift keying system and increasing the data rate to 1200 bit/s. The similar Bell 201 system used both sets of signals (send and receive) on 4-wire leased lines for 2400 bit/s operation.

The next major advance in modems was the Hayes Smartmodem, introduced in 1981 by Hayes Communications. The Smartmodem was a simple 300 bit/s modem using the Bell 103 signaling standards, but attached to a small controller that let the computer send commands to it to operate the phone line.

Broadband Modems

ADSL modems are also a kind of modem, the main difference being that they are not limited to the "voiceband" audio frequencies carried over the telephone line. Recent ADSL modems use coded orthogonal frequency division modulation.

Cable modems are also a kind of modem, this time using a range of frequencies originally intended to carry RF television channels. Multiple cable modems attached to a single cable can use the same frequency band, using a low-level media access protocol to allow them to work together within the same channel. Typically, 'up' and 'down' signals are kept separate using frequency division multiplexing.